1. Field of the Invention
The present invention relates to rechargeable electrical battery systems, and more specifically to systems for automatically protecting against failure of particular cells.
2. Description of Related Art
Electrical power supplies that depend on a number of rechargeable cells connected in series have a great variety of uses, particularly for portable equipment. However, one weakness of these systems is that if a single cell in the series becomes defective it usually acts as a very high resistance or open circuit, making it impossible to efficiently recharge or discharge the remaining series-connected cells. This is particularly undesirable when the power supply is intended for automatic use at a remote location, such as in telemetering the condition of distant oil wells, powering space satellites or similar applications.
To circumvent this problem, a number of ways for automatically detecting and bypassing a cell failure have been developed. For example, to maintain the ability to discharge the remaining operative cells, a separate power rectifier for the anticipated maximum load current is coupled in parallel across each cell, the diode being oriented for reverse-bias when the cell is operating normally. If a cell fails during discharge, the voltage supplied by the remaining operative cells will forward bias the diode, allowing the diode to shunt the discharge current across the single failed cell.
However, adding such rectifiers does not enable the remaining operative cells to be recharged since the recharging current would have to flow in the rectifier's "OPEN" or high resistance direction. Therefore, some bypass circuit is required to effectively shunt any defective cell. For example, Herrin's U.S. Pat. No. 4,774,558, assigned to the same assignee as the present application, requires the use of a shunt rectifier which includes a solder preform that melts when the rectifier shunts a defective cell because of heat given off by the conducting rectifier. The melted preform permanently shorts out the defective cell.
Another way of permanently conductively shorting out a defective cell is described in U.S. Pat. No. 4,935,315 also owned by the assignee of this application. When a light emitting diode in parallel across a defective cell begins to conduct, electromagnetic radiation given off by the diode, such as heat or light, is detected by a sensing circuit. The sensing circuit then closes a latching relay which shunts the defective cell. Although during manufacture the latching relay is initially in the "OPEN" state, once it receives a command to "CLOSE", it remains latched in the "CLOSED" state.
Unfortunately such permanent or irreversible shunting has the disadvantage that should a shunted cell recover, the shunting function cannot automatically be removed. Moreover, the various thermal and mechanical ways of shunting a defective cell have further disadvantages: 1) an identical rectifier structure which includes meltable solder might accidently be damaged when soldering a terminal connection; 2) a relay can fail when exposed to vibration or acceleration; and 3) at high current levels the closure of the fused alloy switch may present high initial contact resistance and high power dissipation leading to switching reliability problems.
Therefore, there is a need for an automatic cell bypass device that supports both charging and discharging of the remaining operative cells yet is reversible, preferably automatically. There is also a need for such a device which operates as a solid state device for greater reliability.